94 . ON HEREDITY. 



which leads one of our butterflies ( Vanessa levana) to lay its green 

 eggs in single file, as columns, which project freely from the stem 

 or leaf, so that protection is gained by their close resemblance to the 

 flower-buds of the stinging-nettle, which forms the food-plant of 

 their caterpillars? 



Of course the butterfly is not aware of the advantage which 

 follows from such a proceeding ; intelligence has no part in the 

 process. The entire operation depends upon certain inherent, ana- 

 tomical and physiological arrangements: on the structure of the 

 ovary and oviducts, on the simultaneous ripening of a certain 

 number of eggs, and on certain very complex reflex mechanisms 

 which compel the butterfly to lay its eggs on certain parts of 

 certain plants. Schneider is certainly right when he maintains that 

 this mechanism is released by a sensation, arising- from the percep- 

 tion (whether by sight or smell, or both together) of the particular 

 plant or part of the plant upon which the eggs are to be laid 1 . 

 At any rate, we cannot, in such cases, appeal to the effects of 

 constant use and the transmission of acquired characters, as an 

 explanation ; and the origin of the impulse can only be understood 

 as a result of the process of natural selection. 



The protective cocoons by which the pupae of many insects are 

 surrounded also belong to the same category, and improvement by 

 practice is entirely out of the question, for they are only constructed 

 once in the course of a life-time. And yet these cocoons are often 

 remarkably complex : think, for instance, of the cocoon spun by the 

 caterpillar of the emperor moth (Saturnia carpini], which i< so 

 tough that it can hardly be torn, and which the moth would be 

 unable to leave, if an opening were not provided for the purpose ; 

 while, on the other hand, the pupa would not be defended against 

 enemies if the opening were not furnished with a circle of pointed 

 bristles, converging outwards, on the principle of the lobster pot, so 

 that the moth can easily emerge, although no enemy can enter. 

 The impulse which leads to the production of such a structure can 

 only have arisen by the operation of natural selection not, of 

 course, during the history of a single species, but during the de- 

 velopment of numerous, consecutive species by gradual and un- 

 ceasing improvements in the initial stages of cocoon-building. 







1 Compare Schneider, ' Der thierische 



